The present invention relates to a system for transmitting and receiving data over a transmission wiring or lines, and more particularly to a data transmission system having a receiving station equipped with a receiving circuit capable of data transmission regardless of variations in load impedance characteristics over the transmission wire to which devices of various impedance characteristics are randomly connected.
FIG. 1 of the accompanying drawings illustrates a general data transmission system comprising a transmitting station 1, a transmission wiring or lines 2, a load 3, and a receiving station 4. Under normal conditions, the load on the transmission wiring 2 is subjected to small variations. Therefore, the presence or absence of a signal can easily be detected by a voltage developed across a load impedance ZR' in the receiving station.
Power wiring transmission is normally employed for reducing the system cost. With a variety of devices connected randomly to power wiring or lines, the level of detected signal voltages across the load impedance becomes quite small if .vertline.ZL'.vertline.&lt;&lt;.vertline.ZS'.vertline. (where ZS' is the load impedance in the transmitting station and ZL' the load impedance). At this time, however, the current flowing through the transmission wiring is substantially at maximum.